A spark plug used for an ignition of an internal combustion engine, such as a gasoline engine, includes a shaft-like center electrode, a cylindrical insulator which holds the center electrode on the inner side thereof, and a metal shell which holds the insulator on the inner side thereof. A substantially L-shaped ground electrode is provided on the metal shell so as to form a spark discharge gap between the ground electrode and center electrode. The metal shell and ground electrode are generally configured from an iron-based material, such as carbon steel, and the outer surface thereof is subjected to a plating treatment for corrosion protection (See Japanese Document No. JP-A-2002-184552).
However, the metal shell has regions, such as an inner wall surface of a cylindrical hole in which is held the insulator and concave portions of the outer surface, through which it is difficult for an electric current to flow in a plating bath, and there is a possibility that the thickness of a plated layer to be formed becomes non-uniformity. When the thickness of the plated layer is non-uniform, it may happen that a stress concentration occurs in a region of the plated layer comparatively small in thickness in a process of crimping the metal shell, or the like, thus accelerating a peeling off of the plated layer. It is an actual situation that no sufficient ingenuity has been exercised so far in suppressing a peeling off of the plated layer by making the thickness of the plated layer uniform, and improving the corrosion resistance of the spark plug.